Hard efforts in endurance rides

I am training for a double century in a little over four weeks (one day STP). I have been doing a mix of hilly and less hilly endurance rides. The STP is quite flat with only about 2000 feet of climbing in 203 miles. I am wondering about the value of putting hard (at or above lactate threshold HR) efforts into my endurance training rides. I have been doing some hard efforts in most rides, but it really has just been going hard up this hill or that hill, nothing focused. I know there is not much training left, but if anyone has any suggestions I would love to read them.

Absolutely! 80-100 mile rides with gaspers are a must-do. I like a continuous hard attack on Union Hill, all the way from the bottom, High Rock and N Cherry Valley all the way to the top, Old W-D, Lord Hill from the west, etc., all ridden all out. Not all on the same ride, but 80 miles with 5000' of hard climbing will really make a difference. Even a couple of rides like that between now and then. I'll also do a few shorter hill sprints, but not really sprints, just one minute hard climbs at redline, like the hill on Ben Howard from either side. Recover at your STP endurance pace between hard climbs.

I did 203 miles at the 12hr Calvin's Challenge with only two training rides longer than 45 miles during the 9 months prior to the race. My secret - short interval rides of an hour or less twice a week. Very hard interval work gives you the fitness to make it easy to cruise at 20 mph. Doing hours and hours of moderate pace endurance work won't be able to give you that with any degree of time effectiveness, if at all. So yeah, add some intensity (and subtract some milage - the trade off is worth it.).

Thanks guys. I will continue with my hill work. I will also try to get more consistent with my midweek hill ride. I should probably up the intensity of that ride. Fortunately, I live in a hilly area so finding good training rides is pretty easy. Time to put the hammer down.